The present invention relates generally to an apparatus for filtering solids from a particulate-laden stream of gas, and more particularly, to an improved gas bypass system and method in a baghouse fabric filter system.
Baghouses are generally employed to separate particulate matter from gas streams, for example, to separate fly ash and other undesirably particulate matter from a boiler flue gas stream, by fabric filtration with fabric filter means generally known as bag filters. Fabric filtration with fabric filter means is not limited to air pollution control, but may also be employed in resource recovery applications where it is an object to recover particulate matter from a gas stream.
The filtration of the particulate matter from particulate-laden gas is carried out in filter houses, known in the trade as baghouses, which include a plurality of fabric bag filters suspended, generally, open-end down, within a plurality of compartments within the baghouse. Particulate-laden gas, such as flue gas containing fly ash particles resulting from the burning of coal, is directed upwardly into each of the bags within a given compartment, such that particulate matter collects inside the bags as a filter cake. Gas is forced to flow through the compartment by either a blower fan or a suction fan, and accordingly, there is a pressure drop across the filters depending upon their resistance to gas flow. As a filter cake accumulates on the bag surfaces, gas flow resistance increases, thereby decreasing gas flow and increasing pressure drop which must then be overcome by the fan. In order to decrease gas flow resistance and thereby increase gas flow and decrease pressure drop, the bag filters or fabric filter means are periodically cleaned to remove the accumulated filter cake.
The particulate-laden gas, for example, derived from the burning of coal, is a hot stream of flue gas directed from a boiler unit or other source by a suitable conduit into a gas inlet manifold of the baghouse filter system. The gas inlet manifold is connected to the gas inlet of a plurality of filter compartments, and the hot, particulate-laden gas is directed into selective gas inlets of a plurality of filter compartments. Generally, means are provided for selectively blocking the gas inlet of each compartment, and the flow of the hot, particulate-laden gas passes through the gas inlet and into the filter compartment in those filter compartments which are not selectively blocked.
Fabric filter collection systems which remove and collect particulate matter from gas streams which are corrosive, explosive and/or operate at high temperatures typically require bypass systems. The gas bypass systems are required to protect the filter bags and/or the process equipment from damage due to system upsets or irregularities. Most of the prior art bypass systems are external to the fabric filter casing, that is, they are located in the conduit carrying the gas stream from the source to the gas inlet manifold, and most generally the prior art flue gas bypass systems are located at the point where the flue gas stream enters the gas inlet manifold. The external gas bypass system permits ease of maintenance due to the fact that there is good accessibility to the isolation dampers regardless of the type of isolation dampers used, for example, butterfly dampers, louvers and the like.
In baghouse filter systems, prior art internal gas bypass systems have been disadvantageous because of inaccessibility of the isolation dampers, and since most dampers commonly used in such systems, for example, butterfly dampers and louvers, require regular maintenance, internal bypass systems have been discouraged because maintenance was virtually impossible except during system shutdowns. The isolation dampers generally require frequent maintenance because the bearings and seals fail due to the heat, dust and the acidic environment, especially when the baghouse filter systems are used in conjunction with the hot, particulate-laden gases derived from boilers.
The external bypass systems have also been disadvantageous because they typically require excessive amounts of real estate for their positioning and location external to the fabric filter casing or baghouse housing. For example, an external bypass system requires as much as 20 to 25 additional feet of space on baghouses used for filtering the flue gas from the boilers of a large power plant. Not only do external gas bypass systems require extra real estate, but they also involve additional expense due to the large amounts of steel plate and due to the insulation of various areas to prevent exposure of certain parts to heat.
Typical baghouse filter construction is shown in U.S. Pat. No. 3,057,137; U.S. Pat. No. 3,898,062; and U.S. Pat. No. 3,945,400. In U.S. Pat. No. 3,898,062 and U.S. Pat. No. 3,945,400, the gas inlet manifold and gas outlet manifold do not share a common wall within the fabric filter casing. In U.S. Pat. No. 3,057,137, the gas inlet manifold and gas outlet manifold have a common wall, and the gas inlet manifold and gas outlet manifold are spaced between the pair of spaced housings, each of which contains a plurality of filter elements, and the housings thus provide a pair of spaced walls which are generally parallel and co-extensive with each other and which are utilized to form, in part, the inlet gas manifold and the outlet gas manifold. In one preferred embodiment of U.S. Pat. No. 3,057,137, the inlet gas manifold and outlet gas manifold are horizontally disposed and are substantially co-extensive in length with the two housings which make up the filter compartments. As described in U.S. Pat. No. 3,057,137, a top wall of the gas inlet manifold is preferably inclined in order to reduce the cross-section of the inlet duct progressively away from the inlet end which is connected to a source of gas to be cleaned. The outlet duct in U.S. Pat. No. 3,057,137 shares a common wall with the inlet gas manifold, so that the top wall of the inlet gas manifold forms the bottom wall of the gas outlet manifold. The outlet gas manifold of the filter system of U.S. Pat. No. 3,057,137 is of progressively increasing cross-section in the direction of gas flow and is of maximum size at the outlet end which is the opposite end of the housing from the inlet end. There appears to be no discussion in U.S. Pat. No. 3,057,137 of a gas bypass system.
A system for removing gaseous pollutants, such as, sulfur oxides, from gases wherein the system has an internal bypass capability is described in U.S. Pat. No. 4,133,659. In U.S. Pat. No. 4,133,659, the gaseous pollutants are removed from gases in one or more beds of activated charcoal formed in a housing along with passages for permitting the passage of the gases directly from the housing inlet to the outlet and passages for permitting the passage of gases from the inlet across the beds of activated charcoal to the outlet. Damper means are associated with the passages for permitting the passage of gases directly from the housing inlet to the outlet, the dampers being movable from an open position in which the gases pass through the first passages and a closed position in which the gases are directed through the second passages, that is, the passages for permitting the passage of gases from the inlet across the bed of activated charcoal to the outlet. In the event of a malfunction of the absorber, that is, the system for removing the gaseous pollutants, or any ancillary equipment connected thereto, damper units can be opened which permits a direct flow of the gases from the inlet through the passages to the outlets and thereafter to the stack, thereby bypassing the beds of charcoal.
Various damper plates as described in U.S. Pat. No. 4,133,659 are used for controlling the flow of gases through the passages, for example, one damper unit includes a single damper plate which is hinged relative to the partition and which can be moved by a wire hoist from an open position in which a corresponding opening formed in the partition is exposed, to a closed position. In another embodiment, a dual-leaf configuration is pivotably mounted relative to the partition and adapted to swing from a open horizontal position to a closed vertical position. It appears that there is no discussion of poppet damper means to direct the gases into the various passages to bypass the columns of activated charcoal or to direct the gases through the columns of activated charcoal. Furthermore, there is no suggestion of the use of a gas outlet manifold having a wall substantially common with the gas inlet manifold over the longitudinal direction of the gas inlet manifold. By examining the teachings of U.S. Pat. No. 4,133,659, it can be seen that the absorber unit for removing gaseous pollutants from gases and which has the internal bypass capability, is not of the type which could be construed as a dust collector system for removing particulate matter from particulate-laden gases because U.S. Pat. No. 4,133,659 teaches the separate use of a dust collector immediately following a boiler unit and thereafter a separate absorber unit for removing the gaseous pollutants after the particulate matter has been removed by a dust collector.
Poppet damper means, for example, poppet valves for baghouse outlet units have been described in the prior art, and in U.S. Pat. No. 4,360,370, there is described the use of poppet valves positioned in openings through which dust-free gas can pass from the filter compartment into a clean gas plenum. The poppet valve comprises a valve plate affixed to a valve stem that is mounted for translational motion in the opening so that the valve plate can cover the opening when the opening is to be closed and can be moved away from the opening when the opening is to be unclosed. There appears to be no suggestion in U.S. Pat. No. 4,360,370 of providing an internal gas bypass system by direct communication between the gas inlet manifold and the gas outlet manifold.
As used herein, a "baghouse" which may also be referred to more generically herein as a "fabric filter system", comprises a plurality of "compartments", also designated as "filter compartments." Each "compartment" is an independent structure, and is normally separated from other compartments within the same baghouse by walls, and is controllably separated from common inlet and outlet gas ducts by dampers or appropriate valves. Each compartment includes multiple bags, typically arranged in rows and columns. The baghouse including the plurality of compartments and the plurality of fabric filter means in each compartment, as well as the gas inlet manifold and gas outlet manifold, is referred to herein as a fabric filter system.